The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services from geosynchronous orbit. A substantial number of radio frequency (RF) filters are required in such spacecraft. For example, a satellite input multiplexor may utilize a number of such filters, each filter having the functionality of separating and isolating a specific respective signal or frequency bandwidth from a broadband uplink signal received by a spacecraft antenna.
A known type of RF filter is a multi-cavity microwave filter having a unitary housing. Each cavity in such a filter may include a dielectric resonator, for example, as described by Cameron, et al., U.S. Pat. No. 5,608,363, or a post, for example, as described by Yu, U.S. Pat. No. 6,882,251, the disclosures of which are hereby incorporated in their entirety into the present application.
FIG. 1A shows an RF filter of the type described by Cameron and Yu. As illustrated in FIG. 1A, RF filter 100 may include a metallic housing 101, in which a number of resonator cavities 120 are formed. Metallic housing 101 is made from an electrically conductive material, such as aluminum. An input port 110 is coupled by probe 111 to a first resonator cavity 120(1). Resonator cavity 120(1) contains an associated dielectric resonator 130(1) and may therefore be referred to as a “loaded” cavity. Alternatively, resonator cavity 120(1) may contain a centrally disposed cylinder (not shown) and be referred to as a coaxial resonator. A sequential series of such resonator cavities 120 may be provided, where the first resonator cavity in the series, 120(1) is proximate to input port 110, and the last resonator cavity in the series, resonator cavity 120(10), is proximate to output port 130. The series of resonator cavities may be arranged, as illustrated, in two rows, such that some non-sequential resonator cavities (for example, 120(2) and 120(9)) are physically adjacent to one another and separated by metal wall 103 of metallic housing 101. Sequential resonator cavities (for example, 120(2) and 120(3)) may be coupled by openings, irises or “slots” 121. Slot couplings 121 are magnetic field-to-magnetic field couplings that may be referred to as “positive couplings”.
Some resonator cavities may be provided with “negative couplings”, and/or positive couplings, by way of electrically conductive coupling probes (hereinafter, “coupling probes”). For example, resonator cavities 120(2) and 120(9) are illustrated as being coupled by coupling probe 140.
Referring now to FIG. 1B, it may be observed that a conventional coupling probe 140 extends through an aperture in metal wall 103 of metallic housing 101. Coupling probe 140 includes first probe end 141 and second probe end 142. First probe end 141 extends into resonator cavity 120(2) of RF filter 100 while second probe end 142 extends into resonator cavity 120(9). In this configuration, coupling probe 140 is capable of providing an energy coupling between resonator cavities 120(2) and 120(9).
Conventionally, an insulating material 105 surrounds a portion of coupling probe 140, as shown in FIG. 1C. Insulating material 105 provides a structural interface between coupling probe 140 and metal wall 103 of metallic housing 101, as well as electrically isolating coupling probe 140 from metal wall 103. As a result of the above-described arrangement, coupling probe 140 “floats” electrically with respect to metallic housing 101.
In a typical communications satellite, space charging phenomena may cause an internal charge buildup on coupling probes that is caused primarily by electrons with energies oscillating between 1 and 50 Kev during, for example, solar ion storms. When charge buildup exceeds a critical value, electrostatic discharge (ESD) events may occur, that represent significant potential threats to sensitive satellite components such as low noise amplifiers, other payload equipment, and spacecraft bus electronics. NASA design guidelines (for example, NASA-HDBK-4002, February 1999) recommend avoidance of “floating conductor” configurations in ESD sensitive components. Some satellite operators prohibit “floating conductors” in general.